In a Coriolis flowmeter, a tube through which a fluid to be measured flows is supported at one end or both ends thereof, and vibration is applied to a portion of the tube around the supporting point in a direction vertical (orthogonal) to the flowing direction of the tube (hereinafter, a tube to which vibration is applied is referred to as a flow tube). The Coriolis flowmeter is a mass flowmeter, which utilizes the fact that the Coriolis forces applied to the flow tube when vibration is thus applied thereto, are proportional to a mass flow rate. The Coriolis flowmeter, which is well known, is roughly classified into a straight tube type and a bent tube type in terms of flow tube structure.
In a Coriolis flowmeter of the straight tube type, when vibration is applied to a straight tube, whose both ends are supported, in a direction vertical (orthogonal) to the straight-tube center portion axis, a difference in displacement due to the Coriolis forces is generated between the support portions and the central portion of the straight tube. In other words, a phase difference signal is obtained, and, based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter thus constructed has a simple, compact, and solid structure. On the other hand, there arises a problem in that it is difficult to achieve high detection sensitivity.
In contrast, the bent tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter from a viewpoint that the bent tube type Coriolis flowmeter allows selection of a shape for effectively obtaining the Coriolis forces. In fact, the bent tube type Coriolis flowmeter is capable of performing mass flow rate detection with high sensitivity. Known examples of the bent tube type Coriolis flowmeter include one equipped with a single flow tube (see, for example, JP 4-55250 A), one equipped with two flow tubes arranged in parallel (see, for example, JP 2939242 B), and one equipped with a single flow tube in a looped state (see, for example, JP 2951651 B).
In a Coriolis flowmeter, forming the flow tube by a single path without being branched off is the best method of solving a problem of clogging in a small diameter sensor. When measuring a fluid having compressibility or discontinuous fluids differing in density and viscosity, it is impossible to effect flow separation in a stable manner if the flow is branched off. From this viewpoint also, it is desirable to form the flow tube by a single flow path. Further, when the flow tube is formed exclusively in one and the same plane (i.e., a single plane), the production of the flow tube is facilitated since the configuration and structure involved are the simplest, which proves effective when reproducibility in form at low cost is required.
However, in a conventional Coriolis flowmeter formed by a single flow path, which does not adopt an opposing construction to cancel vibration, leakage of vibration from the fixed ends to the exterior of the mass flowmeter occurs when the flow tube is vibrated in a tertiary mode or an even-number mode, resulting in generation of zero-point drift or span fluctuation depending upon changes in piping conditions. Further, even in a vibration system using a counter balancer for the purpose of mitigating vibration leakage, a change in vibration leakage is caused by a change in density, resulting in a deterioration in terms of instrumental error property.
As stated above, a counter balancer is usually adopted in order to mitigate vibration leakage. However, in a Coriolis flowmeter utilizing bending vibration of a single tube, the influences of the counter balancer system in terms of density, temperature, and vibration are inevitable.